A genomic library from one clinical isolate of Acinetobacter baumannii was obtained to find genes responsible for tetracycline resistance. Escherichia coli DH5alpha-MDR transformants, selected on Mueller-Hinton agar supplemented with tetracycline 18 mg/L, were thoroughly characterized. In one clone, with an insert of 7,070 bp, it was found that the resistance to tetracycline was mediated by the tet(A) gene (1,200 bp) which encodes a tetracycline efflux pump. This gene was recovered together with tetR(A) (651 bp), the tet(A) repressor. Moreover, the partial sequence (2,008 bp) of a transposase gene, tnpA, and 1,316 bp corresponding to an IS, similar to that described in one strain of Salmonella typhi (IS4321), were found. In this A. baumannii clinical isolate, the tet(A) gene is located in a transposon. The structure of this transposon is similar to that of Tn1721, with the tet(A), tetR(A) and the regions between these genes, being closely related to those of Tn1721. The data indicate horizontal transfer of tetracycline resistance genes between A. baumannii and other genera sharing the same ecological niches.